The US has said President Bashar al-Assad was rapidly "losing control" of Syria and that the international community had to act swiftly to stop the country from descending into an all-out sectarian civil war.

After a day of extraordinary drama on the streets of Damascus, and a bombing which killed at least three senior members of Syria's military hierarchy, Washington said the 16-month uprising against Assad was now entering a defining stage. "It's clear that the Assad regime is losing control of Syria. There is real momentum against Assad, with increasing defections, and a strengthened and more united opposition that is operating across the country," said a White House spokesman, Tommy Vietor.

Vietor said the international community must work with Syria's opposition to craft a workable transition plan.

"The sooner this transition happens, the greater the chance we have of averting a lengthy and bloody sectarian civil war and the better we'll be able to help Syrians manage a stable transition to democracy," he added.

The US defence secretary, Leon Panetta, came to a similarly dark conclusion, describing the situation in Syria as "rapidly spinning out of control". Britain, France and Germany, meanwhile, backed the US's call for an urgent "political transition" in Damascus to a post-Assad government.

Speaking at a Pentagon press conference, Panetta said on Wednesday the Syrian crisis had reached a moment of "real escalation … the violence there has only gotten worse, and the loss of lives has only increased, which tells us that this is a situation that is rapidly spinning out of control."

He said the international community should now bring "maximum pressure" on Assad to step down and allow a transfer of power.

The US defence secretary, Phillip Hammond, speaking with Panetta, described the Syrian opposition as "emboldened" and with "access increasingly to weaponry". There was also "probably some fragmentation around the edges of the regime as well", he added.

The foreign secretary, William Hague, described the situation in Syria as "clearly deteriorating" and repeated his calls for an urgent UN security council resolution – so far blocked by Russia and China.

The security council had been due to vote on Wednesday on a resolution – backed by the UK, the US, France and Germany – to extend the UN observer mission in Syria for 45 days and to place Kofi Anna's peace plan under chapter 7 of the UN charter. Chapter 7 would allow the 15-member council to authorise sweeping diplomatic and economic sanctions as well as military options, something Moscow steadfastly opposes. Annan called for the resolution to be postponed given the rapidly changing events on the ground in Syria.

Angela Merkel, the German chancellor, said it was "high time to ratify the next UN resolution", while the French foreign minister, Laurent Fabius, said it was now "urgent and necessary to find a political transition" that allowed the Syrian people to express its aspirations.

The Kremlin, however, struck a different, and contrary, tone. Speaking to journalist in Moscow, the foreign minister, Sergei Lavrov, conceded the fighting for control of Syria had reached a definitive moment: "A decisive battle is underway," he said. But Moscow unequivocally described Wednesday's deadly bombing as a "terrorist attack" and offered its condolences to "the Syrian people and government" as well as the families of those killed.

Lavrov accused Russia's western partners of inciting a "further escalation" of the conflict, saying: "It is a dead end policy to support the opposition. Assad will not got on his own and our Western partners don't know what to do about that."

The UN secretary general, Ban Ki-moon, was in talks on Wednesday with China's president, Hu Jintao, in Beijing.

Afterwards, Ban said the Chinese leaders agreed the situation in Syria is now "very serious".

"I sincerely hope the members of the security council will be able to discuss with a sense of urgency and take collective action, with a sense of unity," he said. "We cannot go on like this way. So many people have lost their lives during such a long time."

Gala Riani, a Middle East analyst at the Control Risks consultancy, said the suicide bombing was "in some ways the most successful direct attack on the regime we've had so far". She told the Associated Press: "I think the next few days are going to be crucial in signalling where the conflict goes from here.

"At the very least, we can expect the situation to continue to deteriorate. But I think it will take more than this to take the Assad regime down."

The attack at a meeting of high-ranking security officials and ministers in the heart of Damascus will send a message to the top of the Syrian government that it is vulnerable. "It sends a stark message that individual ministers are not safe and is likely to accelerate the erosion of the regime's support base," said Anthony Skinner, head of Middle East consultancy Maplecroft.

The bombing, claimed by the Free Syrian Army (FSA) and also by Islamist group Liwa al-Islam, does not alter the fact that the rebels remain hugely outgunned by Assad's forces, he suggested. "These are very significant developments, but I believe the offensive will be repelled," Skinner said. "Psychologically, though, this will likely give the FSA a significant boost and may also precipitate more defections at a senior level."